Public Service Commissioner Kevin Cramer (R) could upset his party-backed opponent in the Tuesday primary for North Dakota's open at-large seat, a new poll suggested.

Cramer led fellow Public Service Commissioner Brian Kalk, 60 percent to 21 percent with 19 percent undecided, in a Mason-Dixon poll sponsored by the local Valley News Live/KFYR-TV. The poll surveyed 625 likely voters from June 4 to 6. It had a margin of error of 4 percent.

A Cramer victory would make history in North Dakota GOP politics. It's rare for anyone to mount a significant challenge against the GOP's endorsed candidate in a primary — let alone win the GOP nod.

But national conservative activist groups quickly coalesced behind Cramer. FreedomWorks and the Club for Growth, active on behalf of other anti-establishment candidates in several GOP primaries across the country, have backed Cramer and spent on his behalf.